Electromagnetic transponder read terminal operating in very close coupling

ABSTRACT

A terminal for generating an electromagnetic field adapted to cooperating with at least one transponder when the latter enters its field, the terminal including an oscillating circuit adapted to receiving a high frequency A.C. excitation voltage, and circuitry for detuning this oscillating circuit with respect to a determined transmission frequency when a transponder is very close to the terminal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) capable of being interrogated in a contactiess and wireless manner by a unit (generally fixed), called a read/write terminal. The present invention more specifically relates to read terminals of transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal. The present invention applies to such terminals, be they terminals only reading the transponder data, or read/write terminals, which are likely to modify transponder data.

2. Discussion of the Related Art

Electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write unit side. These circuits are intended to be coupled by a close magnetic field when the transponder enters the field of the read/write unit. The range of a transponder system, that is, the maximum distance from the terminal at which a transponder is activated (awake) depends, especially, on the size of the transponder antenna, on the excitation frequency of the coil of the oscillating circuit generating the magnetic field, on the intensity of this excitation, and on the transponder power consumption.

FIG. 1 very schematically shows, in a functional way, a conventional example of a data exchange system between a read/write unit 1 (STA) and a transponder 10 (CAR).

Generally, unit 1 is essentially formed of an oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1, between an output terminal 2 p of an amplifier or antenna coupler 3 (DRIV) and a terminal 2 m at a reference potential (generally, the ground). Amplifier 3 receives a high-frequency transmission signal Tx, provided by a modulator 4 (MOD). The modulator receives a reference frequency, for example, from a quartz oscillator 5 and, if necessary, a data signal to be transmitted. In the absence of a data transmission from terminal 1 to transponder 10, signal Tx is used as a power source only to activate the transponder if said transponder enters the field. The data to be transmitted come from an electronic system, generally digital, for example, a microprocessor 6 (μP).

The connection node of capacitor C1 and inductance L1 forms, in the example shown in FIG. 1, a terminal for sampling a data signal Rx, received from a transponder 10 and intended for a demodulator 7 (DEM). An output of the demodulator communicates (if necessary via a decoder (DEC) 8) the data received from transponder 10 to microprocessor 6 of read/write terminal 1. Demodulator 7 receives, generally from oscillator 5, a clock or reference signal for a phase demodulation. The demodulation may be performed from a signal sampled between capacitor C1 and resistor R1 and not across inductance L1. Microprocessor 6 communicates (bus EXT) with different input/output (keyboard, screen, means of transmission to a provider, etc.) and/or processing circuits. The circuits of the read/write terminal draw the power necessary for their operation from a supply circuit 9 (ALIM), connected, for example, to the electric supply system.

On the side of transponder 10, an inductance L2, in parallel with a capacitor C2, forms a parallel oscillating circuit (called a reception resonant circuit) intended for capturing the field generated by series oscillating circuit L1C1 of terminal 1. The resonant circuit (L2, C2) of transponder 10 is tuned on the frequency of the oscillating circuit (L1, C1 ) of terminal 1.

Terminals 11, 12, of resonant circuit L2C2, which correspond to the terminals of capacitor C2, are connected to two A.C. input terminals of a rectifying bridge 13 formed, for example, of four diodes D1, D2, D3, D4. In the representation of FIG. 1, the anode of diode D1 and the cathode of diode D3 are connected to terminal 11. The anode of diode D2 and the cathode of diode D4 are connected to terminal 12. The cathodes of diodes D1 and D2 form a positive rectified output terminal 14. The anodes of diodes D3 and D4 form a reference terminal 15 of the rectified voltage. A capacitor Ca is connected to rectified output terminals 14, 15 of bridge 13 to store power and smooth the rectified voltage provided by the bridge. It should be noted that the diode bridge may be replaced with a single-halfwave rectifying assembly.

When transponder 10 is in the field of terminal 1, a high frequency voltage is generated across resonant circuit L2C2. This voltage, rectified by bridge 13 and smoothed by capacitor Ca, provides a supply voltage to electronic circuits of the transponder via a voltage regulator 16 (REG). These circuits generally include, essentially, a microprocessor (UP) 17 (associated with a memory not shown), a demodulator 18 (DEM) of the signals that may be received from terminal 1, and a modulator 19 (MOD) to transmit information to terminal 1. The transponder is generally synchronized by means of a clock (CLK) extracted, by a block 20, from the high-frequency signal recovered across capacitor C2 before rectification. Most often, all the electronic circuits of transponder 10 are integrated in a same chip.

To transmit the data from transponder 10 to unit 1, modulator 19 controls a stage of modulation (back modulation) of resonant circuit L2C2. This modulation stage is generally formed of an electronic switch (for example, a transistor T) and of a resistor R, in series between terminals 14 and 15. Transistor T is controlled at a so-called sub-carrier frequency (for example, 847.5 kHz), much smaller (generally with a ratio of at least 10) than the frequency of the excitation signal of the oscillating circuit of terminal 1 (for example, 13.56 MHz). When switch T is closed, the oscillating circuit of the transponder is submitted to an additional damping as compared to the load formed of circuits 16, 17, 18, 19 and 20, so that the transponder takes a greater amount of energy from the high frequency field. On the side of terminal 1, amplifier 3 maintains the amplitude of the high-frequency excitation signal constant. Accordingly, the power variation of the transponder translates as an amplitude and phase variation of the current in antenna L1. This variation is detected by demodulator 7 of terminal 1, which is either a phase demodulator or an amplitude demodulator. For example, in the case of a phase demodulation, the demodulator detects, in the half-periods of the sub-carrier where switch T of the transponder is closed, a slight phase shift (a few degrees, or even less than one degree) of the carrier of signal Rx with respect to the reference signal. The output of demodulator 7 (generally the output of a band-pass filter centered on the sub-carrier frequency) then provides an image signal of the control signal of switch T that can be decoded (by decoder 8 or directly by microprocessor 6) to restore the binary data.

It should be noted that the terminal does not transmit data as long as it receives some from a transponder, the data transmission occurring alternately in one direction, then in the other (half-duplex).

FIG. 2 illustrates a conventional example of a data transmission from terminal 1 to a transponder 10. This drawing shows an example of shape of the excitation signal of antenna L1 for a transmission of a code 0101. The modulation currently used is an amplitude modulation with a rate of 106 kbits/s (one bit is transmitted in approximately 9.5 μs) much smaller than the frequency (for example, 13.56 MHz) of the carrier coming from oscillator 5 (period of approximately 74 ns). The amplitude modulation is performed either in all or nothing or with a modulation ratio (defined as being the difference of the peak amplitudes (a, b) between the two states (0 and 1), divided by the sum of these amplitudes) smaller than one due to the need for supply of transponder 10. In the example of FIG. 2, the carrier at 13.56 MHz is modulated, with a 106-kbit/s rate, in amplitude with a modulation rate tm of, for example, 10%.

FIG. 3 illustrates a conventional example of a data transmission from transponder 10 to terminal 1. This drawing illustrates an example of the shape of control signal VT of transistor T, provided by modulator 19, and of the corresponding signal Rx received by terminal 1. On the transponder side, the back modulation is generally of resistive type with a carrier (called a sub-carrier) of, for example, 847.5 kHz (period of approximately 1.18 μs). The back modulation is, for example, based on a BPSK-type (binary phase-shift keying) coding at a rate on the order of 106 kbits/s, much smaller than the sub-carrier frequency. In FIG. 3, signal Rx has been shown as “smoothed”, that is, without showing the high frequency carrier (for example, at 13.56 MHz) ripple. In the example of FIG. 3, it has been assumed that each of the three shown bits is different from the preceding bit. Thus, for example, a code 010 is transmitted.

It should be noted that, whatever the type of modulation or back modulation used (for example, amplitude, phase, frequency) and whatever the type of data coding (NRZ, NRZI, Manchester, ASK, BPSK, etc.), this modulation or back modulation is performed digitally, by jumping between two binary levels.

The oscillating circuits of the terminal and of the transponder are generally tuned on the carrier frequency, that is, their resonance frequency is set on the 13.56-MHz frequency. This tuning aims at maximizing the energy diffusion to the transponder, generally, a card of credit card size integrating the different transponder components.

As illustrated in FIG. 3, signal VT is formed of a pulse train at the sub-carrier frequency (for example, 847.5 kHz), a phase shift occurring upon each state change from a bit to the next bit. As concerns the signal recovered on the reader side, it appears not to have a “digital” form, which can make its decoding difficult. Indeed, the shape of signal Rx has, for each bit transmission time (9.4 μs), a non-linear increase beginning (in capacitance charge) to a maximum approximately at two thirds of the duration of a bit, then an also non-linear decrease. The enable time, that is, the time taken by signal Rx to reach a level decodable by the demodulator, is linked to the oscillating circuits being tuned. The need for power transfer for the remote supply, associated with the desired system range, requires a high quality factor, and thus that the oscillating circuits be tuned. Now, a high quality factor results in a small pass-band. This results in a limited data flow for the system. Generally, the quality factors are on the order of 10 for the reader and for the transponder.

The transponder may be formed by various objects (key ring, keys, etc.), but is most often, nowadays, in the form of a credit card integrating all the circuits and the antenna or inductance L2. For an information exchange with a reader or a terminal, the card is brought towards antenna L1 of the reader. The distance between the reader and the card varies and, in some applications, a very close or tight coupling transmission is used, the antennas being distant from each other by less than approximately two centimeters. Such a tight coupling transmission may be used, for example, to enable a payment by means of a transponder, and thus guarantee that only the transponder that is closest to the terminal is recognized by said terminal.

A problem that is raised when the oscillating circuits are very close to each other is that, if they are substantially tuned, the power transmitted from the terminal to the transponder is such that the transponder heats up (antenna L2 is generally formed of one or several planar spirals at the card periphery). This thermal effect results in a deformation of the plastic card.

SUMMARY OF THE INVENTION

The present invention aims at providing a novel solution that overcomes the disadvantages of conventional solutions when a transponder is in very tight coupling relation with a read/write terminal.

The present invention aims, in particular, at reducing or minimizing the thermal effect linked to the remote supply of the transponder by the read/write terminal.

The present invention also aims at enabling an increase of the data transmission rate when the transponder is very close to the terminal.

The present invention also aims at providing a solution that requires no structural modification of the transponder.

A feature of the present invention is to detune the oscillating circuit of a read or read/write terminal when a transponder is in very close or tight coupling relation with this terminal.

Document WO-A-98/29760 provides detuning the frequency of an electromagnetic transponder. This document provides for the antenna of a transponder to be “detuned in frequency or mismatched in impedance, so that the transponder and its electronic circuit absorb less radio field and power. Thus, another transponder located in the vicinity of the mismatched or detuned transponder can receive enough of the radio field to operate properly. The transmission system can then detect or consult this other transponder as if it were alone in the field” of the transmitter. Still according to this document, the mismatch means are used “when the transponder is in an unselected state to limit the power and/or field absorption by the transponder in the unselected state”.

The solution advocated by this document amounts to detuning the transponders that are relatively remote from the terminal to maximize the power received by the closest transponder meant to communicate with the terminal. Such a solution does not solve the above-mentioned tight coupling problems. Indeed, the transponder that remains tuned is the selected transponder.

Conversely to this document, the present invention provides a detuned operation in tight coupling. Thus, a feature of the present invention is to provide, for a tight coupling information transmission, a detuned operation of the oscillating circuit of a terminal of a remotely supplied electromagnetic transponder.

The present invention takes account of the fact that the remote supply power recovered on the transponder side is not a monotonic function of the distance that separates the transponder from the terminal.

Indeed, when the oscillating circuits are tuned on the remote supply carrier frequency, if the transponder comes close to a terminal, the remote supply amplitude starts increasing from the system range limit (on the order of some ten centimeters). This amplitude transits through a maximum (critical coupling position) then starts decreasing again when the transponder becomes very close (approximately less than 2 centimeters). For this reason, in particular, it is not provided in conventional systems to make the power of the terminal dependent from the distance at which the transponder is.

The critical coupling position corresponds to the distance at which the coupling between the transponder and the terminal is optimized by a maximum remote supply amplitude received by the transponder when the oscillating circuits of the terminal and of the transponder are both tuned on the remote supply carrier frequency. In other words, the critical coupling frequency corresponds to the distance at which the remote supply power is maximum for a minimum coupling factor, the coupling factor being the ratio of the mutual inductance on the square root of the product of the inductances of the oscillating circuits.

When the oscillating circuit of the terminal is detuned from the remote supply carrier frequency, the power received by the transponder increases as the distance from the terminal decreases, but with a reduced range. In this case, there also is a distance at which the received power is maximum for a given detuning condition. This is an optimal coupling, the critical coupling position being the optimal coupling condition when the two oscillating circuits are tuned on the carrier frequency. It should be noted that the optimal coupling coefficient between the two oscillating circuits depends not only on inductances L1 and L2, on capacitors C1 and C2, and on the frequency (which is here a fixed frequency and corresponds to the carrier frequency), but also on series resistance R1 of the terminal, and on the load of the oscillating circuit of the transponder, that is, on the equivalent resistance of the circuits (microprocessor, etc.) and on the back modulation means (for example, resistor R, FIG. 1), added in parallel on capacitor C2 and on inductance L2. This equivalent resistor will be designated hereafter as R2.

Thus, the contactless and wireless transmission system operates even if one of the oscillating circuits is detuned, provided that the antennas are very close to each other.

More specifically, the present invention provides a terminal for generating an electromagnetic field adapted to cooperating with at least one transponder when the latter enters its field, the terminal including an oscillating circuit adapted to receiving a high frequency A.C. excitation voltage, and means for detuning this oscillating circuit with respect to a determined transmission frequency when a transponder is very close to the terminal.

According to an embodiment of the present invention, the means for detuning the oscillating circuit are formed of the means of current phase regulation in the oscillating circuit with respect to a reference value, the reference value being choosable from among two values, according to whether the oscillating circuit has to operate in tuned or detuned mode.

According to an embodiment of the present invention, the terminal includes means for comparing the phase of the current in the oscillating circuit with a high frequency signal corresponding to a carrier for remotely supplying the transponder entering the electromagnetic field.

According to an embodiment of the present invention, the means for comparing the signal phase in the oscillating circuit with the reference value are formed of a phase demodulator also used for the reception of a signal back modulated by a transponder.

According to an embodiment of the present invention, the terminal further includes means for determining the distance separating a transponder that has entered its field.

The present invention also relates to a contactless and wireless data transmission between a terminal for generating an electromagnetic field according to the present invention and at least one transponder having no independent power supply means.

According to an embodiment of the present invention, the data transmission rate from the transponder to the terminal is different according to whether the oscillating circuit of the terminal is or is not tuned to the determined frequency.

According to an embodiment of the present invention, the data transmission rate from the terminal to the transponder is different according to whether the oscillating circuit of the terminal is or is not tuned to the determined frequency.

The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3, previously described, are meant to show the state of the art and the problem to solve;

FIG. 4 illustrates, in the form of a timing diagram, an embodiment of the transmission method of the present invention in tight coupling;

FIG. 5 partially shows an embodiment of a read/write terminal according to the present invention provided with means for detuning the oscillating circuit of the terminal;

FIG. 6 shows an embodiment of an electromagnetic transponder according to the present invention provided with means for determining the distance separating it from a terminal; and

FIG. 7 partially shows a terminal according to the present invention provided with means for determining the distance of a transponder in its field.

DETAILED DESCRIPTION

The same elements have been referred to with the same references in the different drawings, and the drawings have been drawn out of scale. For clarity, only those elements necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the digital electronic circuits have not been detailed, be it on the transponder or reader side.

A feature of the present invention is to provide detuning the oscillating circuit of a read or read/write terminal of an electromagnetic transponder, when in a tight coupling situation, that is, when their respective antennas are located, for example, at less than 2 centimeters from each other.

The consequence of such a detuning is that the operation becomes similar to that of a transformer, that is, the quality factor is less. Now, in the conventional tuned operation, a quality factor as high as possible is desired, to optimize the power transfer associated with the transponder remote supply.

The fact of detuning the terminal when the coupling is very close has several advantages.

In a detuned operation, the terminal power, that is, the current in the antenna (L1, FIG. 1), may be decreased while transmitting a sufficient power to the card remote supply. Indeed, since the transponder is very close to the terminal, the remote supply range problem has disappeared. The required power then essentially depends on the transformation ratio (ratio between the number of spirals) of the oscillating circuit inductances. The current decrease required in the antenna suppresses the thermal effect on the transponder side.

FIG. 4 illustrates the shape of signal Rx, on the reader side, after a resistive back modulation on the transponder side, while the oscillating circuit of the terminal is detuned from the remote supply carrier frequency (for example, 13.56 MHz). This drawing is to be compared with FIG. 3. As can be acknowledged, the enable times have almost disappeared. Signal Rx has a shape substantially similar to that of the control signal of the gate of transistor T, on the transponder side. Accordingly, it is now possible to increase the transmission rates since it is no longer necessary to wait, for each transmitted bit, that signal Rx reaches the detection threshold of the phase demodulator of the terminal.

It should be noted that the detuning of the oscillating circuit is only desirable in very close or tight coupling. Accordingly, the present invention enables easily dissociating two operating modes of the system according to whether the transponder is or not very close to the reader. It should also be noted that the coupling coefficient is decreased by the detuning of the two circuits. This is not disturbing since they are then very close to each other in transformer-type operation.

An advantage of providing a detuning of the oscillating circuit of the terminal is that this requires no material modification of the transponders, in particularly of existing transponders. This is particularly advantageous to adapt a system without changing all the transponders, which are generally very numerous. Further, if the operating modes in tight coupling and in loose coupling do not need be different, no intervention, not even on the software, is necessary on the transponders. In the opposite case, if the transponders are of read/write type, it is sufficient to reprogram them so that they accept two different operating modes determined by the terminal in the field of which they are likely to be located and that, for example, transmits an information in this direction to the transponder when the tight coupling is detected.

According to the present invention, for the optimal coupling to correspond to the smallest possible distance between the terminal and the transponder, capacitance C1 has to be increased with respect to its tuned value. This amounts to decreasing the resonance frequency of the terminal's oscillating circuit.

FIG. 5 partially shows an embodiment of a read/write terminal 1′ of an electromagnetic transponder, provided with means for detuning the oscillating circuit with respect to the carrier frequency (for example, 13.56 MHz). This embodiment includes varying the value of the oscillating circuit capacitance (C1, FIG. 1).

As previously, terminal 1′ includes an oscillating circuit formed of an inductance or antenna L1, in series with a capacitive element 24 and a resistor R1, between an output terminal 2 p of an amplifier or antenna coupler 3 and a terminal 2 m at a reference potential (generally the ground). Amplifier 3 receives a high frequency transmission signal Tx coming from a modulator 4 (MOD) that receives a reference frequency (signal OSC), for example, from a quartz oscillator (not shown). Modulator 4 receives, if necessary, a data signal to be transmitted. The other components of terminal 1′ are, unless otherwise indicated, similar to those described in relation with FIG. 1.

According to the embodiment of FIG. 5, a phase regulation of the current in antenna L1 with respect to a reference signal REF is performed. This regulation is a regulation of the high-frequency signal, that is, of the carrier signal (for example, at 13.56 MHz) corresponding to signal Tx in the absence of data to be transmitted. This regulation is performed by varying capacitance C1 of the oscillating circuit of terminal 1′ to maintain the current in the antenna in constant phase relation with the reference signal. Signal REF is at the carrier frequency and corresponds, for example, to signal OSC provided by the oscillator (5, FIG. 1) of the modulator.

As illustrated in FIG. 5, capacitive element 24 connected in series with resistor R1 and inductance L1 is controllable by means of signal CTRL, which is provided by a circuit 21 (COMP), the function of which is to detect the phase shift with respect to reference signal REF and to accordingly modify capacitance C1 of element 24.

Variable capacitance 24 may be formed in several ways. Generally, this capacitance must reach a few hundreds of picofarads and withstand, across its terminals, a voltage of more than 100 volts. A diode in which the capacitance of the reverse-biased junction is used as a variable capacitance that depends on this biasing may for example be used. The diode is then connected, by its anode, on the side of reference terminal 2 m and, by its cathode, on the side of inductance L1. As an alternative, a diode-mounted MOSFET transistor may be used. Be it a diode or a MOSFET transistor, the control performed by means of signal CTRL includes modifying the voltage across the diode or the transistor to modify its capacitance.

The phase measurement in the oscillating circuit is performed, for example, from a measurement of the current 1 through this oscillating circuit, symbolized by an element 23. This current measurement element is connected in series with element 24 and inductance L1. The result of measurement MES is sent to phase comparator 21 that then compares the phase of the current measured by block 23 with reference signal REF, and accordingly controls capacitive element 24 by means of signal CTRL.

It should be noted that the phase regulation loop must be sufficiently slow not to disturb the phase modulation at 847.5 kHz, but sufficiently fast as compared to the displacement speed of a transponder in the terminal field, which is generally the displacement speed of a hand. For example, a response time on the order of one millisecond is adequate, the displacement time of a transponder being of several hundreds of milliseconds.

For a detuned operation, the phase reference of comparator 21 may for example be modified to modify the control order of the loop that then regulates on another value, the signals to be compared being then phase-shifted by a value other than 90° in base value. Instead of modifying the reference phase of comparator 21, the bias voltage of element 24 may be shifted, for example, by means of a switched resistor network, controlled by the terminal microprocessor, to make the biasing resistance of element 24 variable.

In the embodiment of FIG. 5, phase comparator 21 is also preferentially used to demodulate signal Rx coming from a possible back modulation of the transponder.

To detune the oscillating circuits when in very close coupling, an information relative to the distance separating the terminal from the transponder is used. This distance may be determined by the transponder and/or by the terminal.

FIG. 6 shows an example of a transponder 30 determining the transponder position with respect to a critical coupling position. As previously, this transponder is formed from a parallel oscillating circuit including an inductance or antenna L2 and a capacitor C2′ between two terminals 11′, 12′ of the circuit.

In the embodiment illustrated in FIG. 6, the rectification performed to extract a D.C. supply voltage Va smoothed by a capacitor Ca is a single-halfwave rectification by means of a diode D, the anode of which is connected to terminal 11′ and the cathode of which is connected to positive terminal 14 of capacitor Ca. Voltage reference 15 corresponds to the negative terminal of capacitor Ca directly connected to terminal 12′. Voltage Va is meant for an electronic block 31 including, for example, circuits 16 to 20 of FIG. 1. A capacitor C3 is mounted in series with a switch (for example, a MOS transistor) K1 between terminals 11 and 12.

The electronic circuit is provided with an input DET receiving local supply voltage Va. Input DET is associated with a circuit (not shown) for measuring voltage Va and with at least one element for storing this measurement. In a specific example of embodiment, this may be a microprocessor (6, FIG. 1).

The storage of the values of the measured voltages is performed either analogically or, preferentially, digitally over several bits, the number of which depends on the desired analysis precision.

The distance determination is performed, for example, as follows. The following measurement cycle is periodically performed when the transponder is in the range of the terminal and, preferably, as soon as the transponder is activated (supplied) by its entering the field of a reader. Transistor K1 is initially on, the oscillating circuit being tuned. The voltage present on terminal DET is measured. Then, transistor K1 is turned off. The circuit is then detuned, its resonance frequency being shifted to, for example, more than twice the tuning frequency if capacitors C2′ and C3 have the same value. The value present on terminal DET is stored again. As an alternative, the first measurement is performed with a detuned circuit. The two obtained values are compared and the result of this comparison is stored, for example on a single bit.

It should be noted that the time (for example, on the order of a few hundreds of milliseconds) required to perform the two “tuned” and “detuned” measurements is small as compared to the transponder displacement speed that corresponds, in most applications, to the displacement speed of a hand.

It should also be noted that the duration for which the oscillating circuit is detuned to perform a measurement is, preferably, chosen to be substantially different from the sub-carrier half-period, so that this measurement is not interpreted by the terminal as a back modulation. Indeed, the detuning of the transponder oscillating circuit translates as a phase shift in oscillating circuit L1C1 (FIG. 1) of the terminal that, during the distance determination, must not be mistaken for a data transmission.

The above measurement cycle is repeated after a short time interval (for example, on the order of one millisecond) that remains fast as compared to the passing time of a transponder before a terminal (several hundreds of milliseconds).

The variation of the value of the comparison bit enables knowing whether the transponder is closer or further away from the terminal as compared to the critical coupling position. If the bit indicates a higher level in detuned position than in tuned position, this means that the transponder is very close to the terminal (in tight coupling). In the opposite case, the transponder is either close to the critical coupling, or between the critical coupling position and the system range limit.

The distance determination with respect to the critical coupling has the advantage that the performed distance determination or area detection (tight coupling or loose coupling) is similar to a differential measurement. Indeed, the detection is performed with respect to the critical coupling that depends on the system and on its environment. Only at the critical coupling is the recovered voltage threshold maximum when the circuits are tuned. It is thus not necessary to provide a specific reference or distance threshold. In other words, the threshold distance between the two tuned and detuned operating modes is then self-adaptive.

Transponder 30 also includes a resistive back modulation circuit formed, preferably, of two resistors R3, R4 respectively in series with a switch K3, K4 between terminals 14 and 15. Resistors R3 and R4 have different values, respectively high and low.

Consider being between the critical coupling and the terminal, resistor R3, which is of high value, is used to perform the back modulation and transistor K1 is turned off. The system then has a detuned operation close to a transformer operation.

Consider being far from the critical coupling position while being further away from the terminal than this position, that is, consider a loose coupling. Transistor K1 is then turned on and the resistive back modulation is performed by means of resistor R4 that is of smaller value. This is then a conventional operating mode.

It should be noted that by using a resistance of small value when away from the terminal, the system range is optimized. The ratio between the respective values of resistors R3 and R4 is, for example, included between 4 and 10 (R3 between 0.4 and 5 kiloohms and R4 between 100 and 500 kiloohms) and, preferably, on the order of 6 (for example, approximately 1500 and 250 ohms).

As an alternative, capacitor C3 used to detune the circuit for the distance determination is also used as a back modulation means. In this case, switched resistors R3, K3, and R4, K4 are eliminated and the values of capacitors C2′, C3 are chosen so that the importance of the detuning is compatible with the phase shift to be detected by the terminal in case of a capacitive modulation. The capacitive modulation directly influences the phase of the voltage across inductance L1 of the terminal without acting upon its amplitude. This eases the phase detection by the terminal. It should be noted that the type of back modulation does not modify the coding, that is, the control signal of the back modulation switch(es) at the carrier frequency.

For a distance determination on the terminal side, a system such a described in WO-A-97/34250 may for example be used. However, according to a preferred embodiment of the present invention, a determination that does not require involving the transponder, that is, that needs no reception of data coming from the transponder is performed.

FIG. 7 partially shows a terminal according to the present invention provided with means for determining the distance of a transponder entering its field. FIG. 7 is based on FIG. 5, of which it only shows capacitive element 24 and current measurement element 2.

According to the embodiment of FIG. 7, the voltage across element 24 is measured by a resistive bridge (resistors R5, R6), the midpoint of which is connected to the anode of a diode D5, the cathode of which is connected to a first input of a comparator (COMP) 40. A capacitor C5 connects the cathode of diode D5 to ground 2 m. Capacitor C5 thus has across its terminals a D.C. voltage, as the peak amplitude of the voltage at the midpoint of bridge R5-R6. A second input of comparator 40 receives a reference voltage Vref. As previously indicated, when the transponder enters the terminal field, the load that it forms varies the current in oscillating circuit L1C1. Since the oscillating circuits are, by default, tuned, the closer the transponder comes, the more the voltage across the capacitive element decreases. The voltage across capacitor C1 (element 24) is indeed equal to the product of the voltage amplitude (constant) provided by the A.C. generator (amplifier 3) by the quality factor. Now, when the distance decreases, the quality factor also decreases. The output of comparator 40 thus indicates the transponder position with respect to a distance threshold (transformed in a voltage threshold Vref). The output of comparator 40 is, for example, sent to microprocessor 6, to switch the system operation to the mode corresponding to the tight coupling and control the detuning or not of the oscillating circuits.

It should be noted that the fact of maintaining the tuned phase constant by means of the regulation loop enables making the distance-impedance characteristic monotonous, that is, without any point of inflexion, and thus to obtain a reliable distance determination.

It should also be noted that, once the distance determination has been performed by the transponder or the terminal, those of the elements which has not participated to the determination can receive the information by a data transmission from the other element. Thus, the choice of the detuning mode (transponder, terminal, transponder and terminal) is independent from the distance determination mode.

Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the sizing of the different resistive and capacitive elements depend on the application and, especially, on the frequency of the various carriers and on the system range. Similarly, the practical implementation of the different circuits of a transponder or a terminal according to the present invention is within the abilities of those skilled in the art based on the functional indications given hereabove.

Further, the choice of the distance determination mode depends on the application and on the element of the system that is preferentially desired to be modified. Preferably, the distance determination is performed, with the detuning, on the terminal side to avoid any modification on the transponder side.

Moreover, it should be noted that the implementation of the present invention is perfectly compatible with conventional modulations. In particular, the data transmission from the transponder to the terminal can always be based on a phase shift keying, be it a tuned or detuned operation. Indeed, the detuning does not change the high frequency carrier frequency (for example, 13.56 MHz) on which the phase shifts are detected at the sub-carrier rate (for example, 847.5 kHz).

One optional modification is to increase the transmission rate when the transponder and the terminal are detuned and in tight coupling.

The present invention is applicable to, among other areas, contactless chip cards (for example, identification cards for access control, electronic purse cards, cards for storing information about the card holder, consumer fidelity cards, toll television cards, etc.) and read or read/write systems for these cards (for example, access control terminals or porticoes, automatic dispensers, computer terminals, telephone terminals, televisions or satellite decoders, etc.).

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto. 

What is claimed is:
 1. A terminal for generating an electromagnetic field adapted to cooperate with at least one transponder when the transponder enters in the electromagnetic field of the terminal, the terminal including: an oscillating circuit adapted to receiving a high frequency A.C. excitation voltage; and means for detuning the oscillating circuit to a detuned mode with respect to a determined transmission frequency when the at least one transponder is very close to the terminal; wherein the terminal provides power to the at least one transponder via the electromagnetic field when the terminal is in the detuned mode.
 2. The terminal of claim 1, wherein said means for detuning the oscillating circuit includes means for regulating a phase of a current in the oscillating circuit with respect to a reference value, the reference value being choosable from among two values, according to whether the oscillating circuit has to operate in tuned or detuned mode.
 3. The terminal of claim 2, further including means for comparing the phase of the current in the oscillating circuit with a high-frequency signal corresponding to a carrier for remotely supplying the transponder entering the electromagnetic field.
 4. The terminal of claim 3, wherein said means for comparing the phase in the oscillating circuit with the reference value includes a phase demodulator also used for reception of a signal back-modulated by a transponder.
 5. The terminal of claim 1, further including means for determining a distance separating a transponder that has entered the electromagnetic field from the terminal.
 6. A contactless and wireless data transmission system between a terminal for generating an electromagnetic field and at least one transponder having no independent power supply means, wherein the terminal is that of claim
 1. 7. The system of claim 6, wherein a data transmission rate from the transponder to the terminal is different according to whether the oscillating circuit of the terminal is or is not tuned to the determined frequency.
 8. The system of claim 6, wherein a data transmission rate from the terminal to the transponder is different according to whether the oscillating circuit of the terminal is or is not tuned to the determined frequency.
 9. A terminal that generates an electromagnetic field, the terminal being adapted to communicate with a transponder when the transponder enters the electromagnetic field, the terminal comprising: an oscillating circuit including a resistive element and a variable capacitive element; a controller, coupled to the oscillating circuit, that switches the oscillating circuit between a tuned mode and a detuned mode; and a first comparator that provides a comparison bit, a value of the comparison bit indicating a distance between the terminal and the remote transponder relative to a critical coupling point; wherein the controller switches the oscillating circuit between the tuned mode and the detuned mode in response to the value of the comparison bit.
 10. The terminal of claim 9, further including: a measuring device that measures a current in the oscillating circuit to provide a measured current; and a second comparator that compares a phase of the measured current with a reference value and generates a control signal; wherein the controller adjusts the variable capacitive element to detune the oscillating circuit based at least in part on the control signal.
 11. A wireless data transmission system comprising: at least one transponder that does not have an independent power source; and a terminal that generates an electromagnetic field, the terminal including: an oscillating circuit having resistive element and a variable capacitive element and adapted to receive an AC excitation voltage; and a controller that switches the oscillating circuit between a tuned mode and a detuned mode based on a distance between the transponder and the terminal; wherein the transponder receives operating power from the terminal via the electromagnetic field when the terminal is in the detuned mode.
 12. The system according to claim 11, wherein the terminal includes a first comparator that provides a comparison bit, a value of the comparison bit indicating a distance between the terminal and the remote transponder relative to a critical coupling point; and wherein the controller switches the oscillating circuit between the tuned mode and the detuned mode in response to the value of the comparison bit.
 13. The system according to claim 11 wherein a data transmission rate between the terminal and the transponder is based, at least in part, on a mode of the oscillating circuit.
 14. The system according to claim 13, wherein the data transmission rate has a first value when the oscillating circuit is in the detuned mode and a second value when the oscillating circuit is in the tuned mode; and wherein the first value is higher than the second value. 